VIVALDI: "THE FOUR SEASONS." Amsterdam Guitar Trio. RCA 5466-2-RC (compact disc). BACH: "BRANDENBURG" CONCERTOS, NOS. 2,3,5,6. Amsterdam Guitar Trio; Tini Mathot, harpsichord. RCA 6546-2-RC (compact disc). Guitar ensembles are becoming increasingly popular concert attractions, and Baroque transcriptions fill up much of the repertory list. These short discs--the "Four Seasons," for example, last only 42 minutes--could have included some of the trio's exciting modern repertory, but amply demonstrate the group's individual and ensemble brilliance, in cool, quick readings. The Vivaldi concertos sound more idiomatic in this format than do the Bach pieces, all arranged by members of the ensemble. The sound of the guitars, however, works very well with that of the harpsichord in the Fifth "Brandenburg."